This invention relates to fiber optical feedthroughs, and methods of securing an optical fiber within a bulkhead.
Fiber optical feedthroughs are used for transmitting and receiving data in deep drilling operations, such as drilling for oil wells and gas wells. In these types of operations, the feedthroughs can be exposed to extreme downhole environments having high pressures and high temperatures, e.g., up to 30,000 psi and 300xc2x0 C. Furthermore, the feedthroughs are commonly exposed to these extreme environments for extended periods of time.
In operation, the feedthroughs are often secured to a bulkhead assembly. Materials, e.g., epoxies, that typically can be used to assemble a feedthrough to a bulkhead may not survive the extreme downhole environment. Even if they can survive the high pressures and temperatures, these materials may survive only for a short period of time.
We have developed an improved method of securing and sealing a metal-clad optical fiber within a bulkhead, which can result in a seal capable of withstanding extreme temperatures and pressures over an extended period of time. Metal-clad fibers are mechanically robust, and the cladding layer can provide a hermetic seal over the glass fiber. We have realized, in the development of this method, that soldering directly to some metal cladding layers commercially available in optical fibers can result in a diffusion of the cladding material into the solder material. Diffusion of the cladding material exposes the bare glass of the optical fiber. Bare glass can create weak spots on the fiber, thereby making handling difficult, and can make the assembly more susceptible to premature failure due to ingress of moisture and hydrogen. Our method minimizes the exposure of bare glass by providing the fiber with a diffusion barrier over its metal cladding and forming a layer of solder-compatible material over the diffusion barrier, such that the fiber may be securely soldered into a housing without significant alteration (e.g., by diffusion) of the cladding material adjacent the glass core of the fiber.
In one aspect, the invention features a method of securing and sealing an optical fiber within a bulkhead. The method includes forming a layer of a solder-compatible material over a diffusion barrier covering a metal-clad optical fiber; extending the optical fiber through a metallic housing, with a portion of the layer of solder-compatible material within the housing, the housing and the solder-compatible material defining a gap therebetween; and bridging the gap with a solder to secure the fiber to the housing and to form a pressure-tight seal between the fiber and the housing.
Embodiments of the invention may include one or more of the following features. The method further includes placing a solder pre-form in the housing. The method further includes threading the optical fiber through the pre-form. The method further includes forming a second layer of a solder-compatible material in the housing. Bridging the gap includes resistively heating the solder. The method further includes ultrasonically cleaning the fiber and the housing after resistively heating the solder. The method further includes forming a protective layer over the secured optical fiber and housing, by a plating.
In another aspect, the invention features an optical feedthrough having a metallic housing; an optical fiber disposed in the housing, the fiber having, over a length adjacent the housing, a metallic layer covering the optical fiber, a diffusion barrier about the metallic layer, and a layer of a solder-compatible material about the diffusion layer; and a solder disposed in the housing and connecting the fiber to the housing, the solder bridging a gap between the metallic housing and the solder-compatible material to form a pressure-tight seal about the fiber.
Embodiments of the invention can include one or more of the following features. The metallic cladding layer includes gold. The diffusion barrier includes nickel. The solder-compatible material includes gold. The solder includes silver. The solder is connected to a layer of a solder-compatible material on the housing. The solder-compatible material on the housing includes gold. The connected housing and fiber are coated with an exterior protective layer. The protective layer includes gold. The optical feedthrough is adapted to be exposed to at least 20,000 psi of pressure across the solder at 300xc2x0 C. without leakage.
The resulting feedthrough is resistant to changes in temperature and pressure, and can be constructed to withstand differential pressures greater than 30,000 psi at temperatures up to 300xc2x0 C. With a post-assembly, gold-plating process, the feedthrough is further resistant to corrosion. The feedthrough can be installed for long periods of time (up to 5-10 years) in industrial environment, and is particularly suitable for use downhole or in sub-sea oilfields. The optical feedthrough also exhibits low optical losses, e.g., less than about 0.3 db, which are not adversely affected by changes in temperature and pressure.
The above-described method of securing and sealing the fiber to the housing is simple, inexpensive, and produces high yield. No stripping of the fiber to bare glass is required during manufacture, which can damage the fiber and make it susceptible to breaking during handling. The gold cladding layer found on some optical fiber is protected from dissolution during soldering by the diffusion barrier. Controlled resistance heating of a solder pre-form avoids heat damage to the fiber. The method can be applied to a wide variety of materials with proper choices of fluxes and solders.